Screening Workflow to Segregate Seepage Vs. Non-Seepage Derived Sources of Mature Petroleum in Seabed Core Samples: Example from a Deep Water West African Basin

Andrew Pepper¹, Cait Keegan², and Gunardi Sulistyo^1
1Exploration, Hess Corporation, Houston, TX
²KGC Consulting, Stavanger, Norway

Seep detection, via geochemical analysis of seabed core materials, continues to be an important tool in the regional evaluation of frontier basins / plays and in the pre-drill evaluation of prospects. The mobilization and execution of such surveys is costly; and dry holes encouraged by false positive seep indications even more so. Therefore, workflows designed to detect real seepage vs. other non-seepage related sources of sedimentary petroleum are of great potential value to the exploration team.

In this paper we summarize a seabed core screening workflow using Hess’ actual evaluation of a West African deep-water Petroleum System.

Conventionally, bulk solvent extracts from seabed core materials are first screened by inexpensive fluorescence techniques, followed by GC and then GC-MS on the considered “most likely” seep candidates. Samples with the GC and GC-MS characteristics of mature petroleum are regarded as evidence of “real” seepage. While this is an effective way of screening out immature recent organic matter (ROM) from mature petroleum, our experience in many basins shows it is ineffective at screening out re-worked - i.e. detrital - organic matter (RWOM): a solvent extract from mature RWOM - i.e. sediment containing eroded fragments of a source rock previously matured during a previous basin cycle - will closely resemble expelled petroleum in its GC and GC-MS character.

To distinguish the two, we have found liquid chromatography data invaluable. Fractionation during petroleum release from kerogen imparts a very different SARA composition to the expellate: expelled petroleums (i.e. “real” seeps) have very a different SARA composition vs. that of RWOM.

Conventional assessments of petroleum seepage continue to result in many false positive results. SARA screening as part of an industry-standard workflow would greatly improve matters.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery